Various types of vessels and platforms are used in the offshore industry, either for stationary operation or designed to permit transfer between different fields.
Certain of the vessels of the last mentioned type are "semi-submersible", where an operating platform is carried by pontoons subdivided into ballast tanks, which makes it possible to raise the platform so the vessel during transfer (transit position) will move with the decks of the pontoons about at water level. When the vessel has reached the desired position the pontoons will be submerged to a position well below the water level, so the vessel will not noticeably suffer from the impact of wave movements.
The present invention refers to a semi-submersible offshore vessel of the kind having two parallel pontoons and an operating platform carried by the pontoons by means of at least two pairs of vertical columns. Usually the vessel is provided with two pairs of columns only, which then have to be very sturdy. The fact that the pontoons will be lowered to a considerable depth, while the operating platform still remains well above the water level means that the columns must be very high.
Earlier types of these vessels were provided with a considerable number of stays interconnecting the columns as well as running between the latter and the platform. By designing the latter as a strong box structure it has been possible to reduce the number of stays considerably. It has thus been proposed to use a single horizontal, transverse brace between two columns located side by side.
Heavy waves hitting the vessel obliquely in relation to a longitudinal middle plane will however tend to displace the pontoons lengthwise in relation to each other.
If the vessel is provided with four columns those may be regarded as located at the corners of a rectangular figure, where the pontoons form two sides, and the cross braces from the other two sides. Subjected to obliquely directed forces form the waves, the rectangular figure tends to be transferred into a rhombus, which will impose heavy strains upon the whole structure.